Noise and rumble filter has LP filter and voltage divider - are designed to share one resistor to reduce number of components required

摘要

The noise and rumble filter has two transistor amplifier stages connected together by high pass and low pass filters. The resistor (20) belonging to the low pass filter also forms part of the resistive voltage divider (20, 21, 22) that fixes the working point of the second transistor stage (24). The base of the first transistor stage (15) is d.c. coupled over an impedance (13, 29) to the emitter of the second transistor stage. The advantage of the noise filter lies in its reduced number of components -- i.e. by using the LP filter's resistor for voltage division instead of earthing it as normal.